1. Field of the Invention
The present invention relates to a laser resonator which is used in, for example, a solid laser device installed in airports or the like, or a solid laser device mounted in trajectory bodies such as artificial satellites or aircrafts, and more particularly which requires a long resonance length for a pulse type coherent lidar (laser rader) for measuring the wind velocity or the like.
2. Description of the Related Art
For a coherent lidar for measuring the wind velocity or the speed of a movable body, in order to transmit the laser beam towards the fields, the wavelength in the range of 1.4 μm to 2.0 μm (the eye safe wavelength) which is safe for human eyes is required.
As for the laser material for oscillating with this eye safe wavelength, for example, there have been well known Er: Glass (the oscillation wavelength is 1.5 μm), Er, Yb: Glass (the oscillation wavelength is 1.5 μm), Er: YAG (the oscillation wavelength is 1.6 μm), Tm: YAG (the oscillation wavelength is 2 μm), Tm, Ho: YAG (the oscillation wavelength is 2 μm), Ho: YLF (the oscillation wavelength is 2 μm), Tm, Ho: YLF (the oscillation wavelength is 2 μm), and the like. In general, however, in each of the laser materials, the gain inherent in the material is small.
In this case, in order to obtain the output with high efficiency, it is required that the cross sectional area in the direction perpendicular to the optical axis of the laser material is reduced to carry out the excitation with high density by an excitation light source to obtain the relatively high gain. In addition, for the coherent lidar, in order to enhance the accuracy of measuring the wind velocity, the long pulse width tp is required. Moreover, in order to reduce a divergence angle of outputted laser beams, a high beam quality is required.
When a small signal gain of the laser material is g0l, a circulation loss in the resonator is loss and the light velocity is c, in the standing wave type laser resonator, the efficiency η of taking out the energy, and the pulse width tp when the output becomes the maximum are expressed as Expression (1) and Expression (2), respectively.                               η          =                      1            -                          (                                                1                  +                                      ln                    ⁢                                                                                   ⁢                    z                                                  z                            )                                      ⁢                                  ⁢                  z          =                                    2              ⁢                              g                0                            ⁢              l                        loss                                              (                  Expression          ⁢                                           ⁢          1                )                                                      t            p                    =                                                    2                ⁢                L                                            c                ·                loss                                      ⁢                          (                                                ln                  ⁢                                                                           ⁢                  z                                                  z                  ⁡                                      [                                          1                      -                                              a                        ⁡                                                  (                                                      1                            -                                                          ln                              ⁢                                                                                                                           ⁢                              a                                                                                )                                                                                      ]                                                              )                                      ⁢                                  ⁢                  a          =                                    (                              z                -                1                            )                                      z              ⁢                                                           ⁢              ln              ⁢                                                           ⁢              z                                                          (                  Expression          ⁢                                           ⁢          2                )            
Now, since the condition in which the laser oscillation occurs becomes 2 g0l>loss, i.e., z>1, the extraction η is increased as g0l is further increased. Therefore, in order to obtain the pulse width tp with the high efficiency, the high small signal gain g0l and the long resonator length L are both required.
A loss to a propagation mode in the laser resonator can be described qualitatively using a Fresnel number N represented by a smallest aperture radius “a” in the resonator and a resonator length L. The Fresnel number N is defined by the following expression:N=a2/λL  Expression (A1)
FIG. 23 shows a relationship between the Fresnel number N and a loss given to the propagation mode in the laser resonator, which is described in “Springer Series in Optical Sciences Vol. 1 ‘Solid-State Laser Engineering Ver. 4’ Walter Koechner (1995, Springer, Germany), page 202”.
As the Fresnel number decreases, the loss given to the propagation mode increases. A loss generated due to a transmissivity of optical components, or the like of a general laser resonator is in the order of several percent to 10%. In the case in which the Fresnel number N is smaller than 0.7, a loss given to a basic mode (TEM00 mode) that is least affected by a loss increases to the same degree as other losses, whereby fall in an output occurs and it becomes difficult to obtain stable laser oscillation.
On the other hand, since a loss to a higher mode also decreases when the Fresnel number increases, oscillation of a higher mode is generated and it becomes difficult to obtain a laser output of a high beam quality. Thus, it is desirable to set the Fresnel number to the order of three at the maximum. In a laser apparatus used in a coherent lidar, since a cross-section in a direction perpendicular to an optical axis of a laser material is required to be reduced, an aperture radius “a”, which gives the Fresnel number N of a resonator, is generally restricted by a radius of the laser material.
The conventional laser resonator will herein below be described with reference to the associated ones of the accompanying drawings. FIG. 18 is a schematic view showing the construction of a conventional laser resonator which, for example, is shown in an article of “Springer Series in Optical Science”, by Walter Koechner, Solid-state laser engineering, 4th edition (Springer, Germany, 1995, pp. 197), and an article of “LASERS”, by Siegman (University Science Books, U.S.A., 1986), pp. 755.
In FIG. 18, reference numerals 1 and 2 denote concave reflecting mirrors which are arranged at a distance of the resonator L in such a way as to face each other to confine therein a laser beam, reference numeral 3 denotes a laser material, reference numeral 4 denotes an excitation light source for exciting the laser material 3, reference numeral 5 denotes a resonance mode of the Gaussian beam which is defined between the concave reflecting mirrors 1 and 2, reference numeral 6 denotes an aperture for limiting the resonance mode, and reference numeral 7 denotes an optical axis.
Next, the operation of the conventional laser resonator thus described will hereinbelow be described with reference to the associated ones of the accompanying drawings.
In the laser resonator as described above, the laser beam makes a round trip between the concave reflecting mirrors 1 and 2 to pass repeatedly through the laser material 3 which is excited by the pump light source 4 to be optically amplified, thereby forming the resonance modes 5 of the Gaussian beam. The aperture 6 is arranged in order to select only the low-order mode of the resonance mode 5 to generate the laser beam of high quality. As the aperture 6, the aperture of the optical component for the laser material 3 and the mirrors which are arranged within the laser resonator may be employed in some cases.
When the curvature of the concave reflecting mirror 1 is R1, the curvature of the concave reflecting mirror 2 is R2, and the distance between the concave reflecting mirrors 1 and 2 is L, a beam size ω0 in the position where the beam size (1/e2radius) of the resonance mode 5 within the laser resonator, a beam size ω1 of the resonance mode 5 in the concave reflecting mirror 2, and the beam size ω2 of the resonance mode 5 in the concave reflecting mirror 2 are expressed as the following Expression (3).                                                                         ω                0                4                            =                                                                    (                                          λ                      π                                        )                                    2                                ⁢                                                                            L                      ⁡                                              (                                                                              R                            1                                                    -                          L                                                )                                                              ⁢                                          (                                                                        R                          2                                                -                        L                                            )                                        ⁢                                          (                                                                        R                          1                                                +                                                  R                          2                                                -                        L                                            )                                                                                                  (                                                                        R                          1                                                +                                                  R                          2                                                -                                                  2                          ⁢                          L                                                                    )                                        2                                                                                                                                          ω                1                4                            =                                                                    (                                                                  λ                        ⁢                                                                                                   ⁢                                                  R                          1                                                                    π                                        )                                    2                                ⁢                                                                            R                      2                                        -                    L                                                                              R                      1                                        -                    L                                                  ⁢                                  (                                      L                                                                  R                        1                                            +                                              R                        2                                            -                      L                                                        )                                                                                                                        ω                2                4                            =                                                                    (                                                                  λ                        ⁢                                                                                                   ⁢                                                  R                          2                                                                    π                                        )                                    2                                ⁢                                                                            R                      1                                        -                    L                                                                              R                      2                                        -                    L                                                  ⁢                                  (                                      L                                                                  R                        1                                            +                                              R                        2                                            -                      L                                                        )                                                                                        (                  Expression          ⁢                                           ⁢          3                )            
Now assuming that for the sake of simplicity, the relationship of R1=R2=R3 is established, the above Expression (3) is rewritten into the following Expression (4).                                                                         ω                0                4                            =                                                                    (                                          λ                      π                                        )                                    2                                ⁢                                                      L                    ⁡                                          (                                                                        2                          ⁢                          R                                                -                        L                                            )                                                        4                                                                                                                        ω                1                4                            =                                                ω                  2                  4                                =                                                                            (                                                                        λ                          ⁢                                                                                                           ⁢                          R                                                π                                            )                                        2                                    ⁢                                      (                                          L                                                                        2                          ⁢                          R                                                -                        L                                                              )                                                                                                          (                  Expression          ⁢                                           ⁢          4                )            
That is, the beam size ω0 in the position where the beam size of the resonance mode 5 becomes the smallest is decreased the smaller the resonance length L, and also as smaller the value of 2×R−L. in this connection, when it is assumed that the relationship of R1=R2=R3 is established, the position of ω0 where the beam size becomes the smallest, becomes the center of the laser resonator, i.e., the position which is L/2 away from the concave reflecting mirror 1. From Expression (4), in order to obtain the small ω0, there are required the short resonance length L and the concave reflecting mirror having the curvature R with which the value of 2×R−L becomes small.
FIG. 19 shows the relationship between the curvature R of the concave reflecting mirror, and ω0 and ω1 when the resonance length L is set to 2 m. For example, the curvature R with which the value of ω0 equal to or smaller than 0.25 mm is obtained is in the range of 1.017 m to 1.000 m, and the value of ω1 at this time is in the range of 1.92 mm to . Therefore, when the beam size is made to be small, the allowable range of the curvature R is narrow and also the beam size ω0 becomes sensitive to the change in the curvature R and the resonator length L. In addition thereto, the operation of the resonator becomes easily unstable, and hence the adjustment thereof becomes difficult. Also, since when the resonator length L is made longer with the same beam size, the operation of the resonator becomes further unstable, thus it is impossible to make the resonator length L sufficiently long.
In addition, in the case in which a radius in a direction perpendicular to an optical axis of the laser material 3 is assumed to be “a”, in order to suppress a higher mode without significantly increasing a loss to a basic mode, in general, a size of the laser material 3 is set such that the aperture size “a” is approximately 1.5 times as large as the beam size ω0. The Fresnel number N of this conventional laser resonator is 0.047. In this case, from FIG. 23, a large loss to the basic mode is generated and it becomes difficult to obtain stable laser oscillation.
Next, FIG. 20 is a schematic view showing the situation of the resonance beam mode when an inclination δ occurs in he concave reflecting mirror 1. When an inclination occurs in he concave reflecting mirror 1, the misalignment occurs between he optical axis 8 of the resonance mode 5 and the optical axis 7 f the laser resonator. At this time, when the deviation angle of the optical axis in the concave reflecting mirror 1s d1, the deviation angle of the optical axis in the concave reflecting mirror 1 is δ1, the deviation amount of optical axis in the center of the resonator is d0, and the deviation angle of the optical axis in the center of the resonator is δ0, d0, δ0, d1, and δ1 are respectively expressed on the basis of Expression (5).                                                                         d                0                            =                                                θ                  ·                  R                                2                                                                                                        θ                0                            =                                                θ                  ·                  R                                                                      2                    ⁢                    R                                    -                  L                                                                                                                        d                1                            =                                                θ                  ·                  R                  ·                                      (                                          R                      -                      L                                        )                                                                                        2                    ⁢                    R                                    -                  L                                                                                                                        θ                1                            =                                                θ                  ·                  R                                                                      2                    ⁢                    R                                    -                  L                                                                                        (                  Expression          ⁢                                           ⁢          5                )            
FIG. 21 shows the relationship between the curvature R of the concave reflecting mirror, and the deviation amount d0 with the optical axis in the center of the resonator and the deviation amount d1 with optical axis in the concave reflecting mirror 1 when the resonator length is 2 m and when giving the concave reflecting mirror 1 an inclination of θ=100 μrad. In this connection, at the same time, FIG. 21 also shows the relationship between the curvature R of the concave reflecting mirror, and ω0 and ω1 shown in FIG. 19. The deviation amount d1 in the position of the concave reflecting mirror 1 is increased as the mode size θ0 is further decreased. For example, when θ0=0.25 mm, d1=−2.9 mm.
FIG. 22 shows the relationship between the curvature R of the concave reflecting mirror and the deviation angle θ0 (=θ1) with the optical axis in the central position of the resonator when the resonator length L is 2 ms and when giving the concave reflecting mirror 1 the inclination of θ=100 μrad. The deviation angle θ0 in the central position of the resonator is increased as the curvature R of the concave reflecting mirror 1 approaches 1. For example, when ω0=0.25 mm, ω0=3.0 mrad is obtained. This means that the inclination occurs which is 30 times as large as the inclination angle θ=100 μrad of the mirror.
Since when the resonance mode in the position of the concave reflecting mirror 1 is deviated, the eclipse of the apertures of the optical components arranged in the aperture 6 and on the optical axis occurs, the laser output is decreased and also the quality of the laser beam is degraded. For example, when ω0=0.25 mm, the beam size ω1 in the position of the concave reflecting mirror 1 is 1.92 mm. Thus, when the above-mentioned deviation θ=100 μrad occurs, the resonance mode 5 is perfectly refused by the aperture 6, therefore the resonance mode cannot be formed and the laser output cannot be obtained. In addition, the slight inclination of the concave reflecting mirror gives the optical axis of the resonance mode within the resonator a large inclination so that the stability in the emission direction of the outputted laser beam is reduced.
In addition, in order to make the polarization of the laser beam linear, e.g., in the case when the emitted laser beam is used in the wavelength conversion, a polarizer needs to be arranged on the optical path. When the polarizer is arranged on the optical path of the laser beam, the degradation of the extinction ratio of the polarizer occurs depending on the incident angle of the laser beam. For this reason, when the angular deviation of the laser beam is large, a part of the laser beam is outputted to the outside by the polarizer so that the efficiency of utilizing the laser beam is reduced.
Furthermore, when a birefringent material such as an EO-Q switch is arranged on the optical path of the laser beam, the birefringent material changes the polarization of the laser beam depending on the incident angle of the laser beam. Therefore, the resonance cannot be carried out as the laser beam so that the efficiency of utilizing the laser beam is reduced.
In the conventional laser resonator as described above, if the beam size ω0 of the resonance mode is made small in the long resonator length, then the change in the resonator mode 5 is increased with respect to the change in the curvature R of the concave reflecting mirror and the resonator length L. Therefore, there arises the problem that the operation of the laser resonator becomes easily unstable and hence the adjustment thereof becomes difficult.
In addition if the resonator length L is wanted to be made long with the same beam size, the operation of the laser resonator becomes further unstable. Therefore, there arises the problem that the resonator length L cannot be made sufficiently long.
In addition, since the Fresnel number N of the resonator decreases, there arises a problem that a loss given to the basic mode is large and stable laser oscillation cannot be obtained.
Furthermore, when the inclination occurs in the concave reflecting mirror 1, the eclipse due to the aperture 6 and the aperture(s) of the optical component(s) arranged on the optical axis occurs. Therefore, there arises the problem that the laser output is reduced, the efficiency of utilizing the laser beam is reduced and the quality of the laser beam is degraded.
In addition, since the slight inclination of the concave reflecting mirror 1 gives the resonance mode within the resonator the large inclination, there arises the problem that the stability in the emission direction of the outputted laser beam is reduced.
Further, when the polarizer is arrange on the optical path of the laser beam, the extinction ratio of the polarizer is degraded depending on the incident angle of the laser beam. Therefore, there arises the problem that a part of the laser beam is outputted to the outside by the polarizer and hence the efficiency of utilizing the laser beam is reduced.
Furthermore, when the birefringent material such as the EO-Q switch is arranged on the optical path of the laser beam, the birefringent material changes the polarization of the laser beam depending on the incident angle of the laser beam. Therefore, there arises the problem that the resonance cannot be carried out as the laser beam and hence the efficiency of utilizing the laser beam is reduced.